As a conventional snap ring attachment, as shown in FIG. 1A to FIG. 1C, there is known a snap ring attachment including a guide cylinder 1 that defines a guide path 1a having a circular cross section to guide a snap ring SR while directly coming into contact with a piston P, a pushing member 2 having a discoid pressing portion 2a to push the snap ring SR contracted in a diametric direction and introduced in the guide cylinder 1 toward an attachment groove Pg, and others when attaching the snap ring SR to the attachment groove Pg of the piston P in a state that a piston pin Pi is inserted in a pin hole Ph of the piston P.
Further, when the snap ring SR is contracted in the diametric direction and supplied into the guide cylinder 1 as shown in FIG. 1A, a pushing rod 2 is moved in a state where the pressing portion 2a is in contact with the snap ring SR as shown in FIG. 1B, whereby the snap ring SR is attached to the attachment groove Pg as shown in FIG. 10.
Meanwhile, as the piston P which is an attachment target, various types of pistons are present in accordance with types of engines, and hence an end region of the guide cylinder 1 must be also formed into a shape associated with the piston P. In particular, in case of the piston P having a configuration that a distance between an annular groove Pcg to which the piston ring is attached and the attachment groove Pg is small, a notch portion 1b is provided in an end outer peripheral region of the guide cylinder 1 to adapt the guide cylinder 1 to the piston P.
However, when the notch portion 1b is provided to the guide cylinder 1 in this manner, a thickness of a wall that defines the guide path 1a is reduced, the snap ring SR (an edge of a gap in particular) that slides on the guide path 1a to move may possibly abrade a wall surface to break the guide cylinder 1.
As another snap ring attachment, there is known a snap ring attachment including a guide cylinder that defines a guide path having a circular cross section tapered into a conical shape on an end side so as to guide a supplied snap ring to an attachment groove of a piston while pressing the snap ring and contracting the snap ring in a diametric direction, a pushing member that is inserted into the guide cylinder to allow its reciprocating and rotating motions and has an inclined surface for pressing the snap ring and a cylindrical protruding portion configured so as to be inserted into a through hole of a piston pin, and others, wherein the snap ring attachment has a configuration that the pushing member is pushed in to move the inclined snap ring while contracting the snap ring in the diametric direction, and the pushing member is rotated when a part of the snap ring reaches the attachment groove, whereby the entire snap ring is attached to the attachment groove (see, e.g., Patent Document 1).
However, in this attachment, an operator manually performs all of an operation of supplying the snap ring into the guide cylinder, a pushing operation, a radially contracting operation, and an attaching operation based on rotation, and the attaching operation is not automatically carried out. On the other hand, when driving this pushing member by, e.g., an actuator to achieve automation in this attachment, a reciprocation driving actuator configured to push in the pushing member and a rotation driving actuator configured to rotate the pushing member are required, thereby resulting in a complicated configuration.
Further, still another snap ring attachment, there in known a snap ring attachment including an outer sleeve that comes into contact with one end side of a pin hole of piston in a state that a snap ring is inserted therein and horizontally held, an inner sleeve that is slidably provided in the outer sleeve to press the snap ring, an insertion rod that is inserted into the pin hole from the other end side of the pin hole of the piston and has an end surface for erecting and positioning the snap ring held in the outer sleeve, and others, wherein the inner sleeve and the insertion rod abut on each other from both sides, the snap ring is held and moved in an upright state by an end surface of the inner sleeve and the end surface of the insertion rod, and the snap ring is attached to an attachment groove of the piston (see, e.g., Patent Document 2).
However, in this attachment, since the snap ring is attached to one of the attachment grooves alone in a state that the piston pin is not inserted in the pin hole of the piston, the snap ring cannot be attached to the other attachment groove after insertion of the piston pin. Furthermore, in order to attach one snap ring, the outer sleeve in which the inner sleeve is accommodated and the insertion rod must be arranged on both sides between where the piston is located, thereby leading to an increase in size of the attachment.    Patent Document 1: Japanese Utility Model KOKOKU Publication No. 05-18060    Patent Document 2: Japanese Unexamined Patent Publication No. 02-24028